Synthesis and Photodimerization of 2- and 2,3-Disubstituted Anthracenes: Influence of Steric Interactions and London Dispersion on Diastereoselectivity

There
is increased evidence that the effect of bulky groups in
organic, organometallic, and inorganic chemistry is not only repulsive
but can be attractive because of London dispersion interactions. The
influence of the size of primary alkyl substituents in 2- and 2,3-positions
of anthracenes on the diastereoselectivity (anti vs syn dimer) of
the [π4s + π4s] photoinduced dimerization
is investigated. The synthesis of the anthracene derivatives was achieved
by Suzuki–Miyaura reaction of 2,3-dibromoanthracene with alkylboronic
acids as well as by reduction of anthraquinones that were obtained
from 2,3-disubstituted 1,3-butadienes and naphthoquinone followed
by dehydrogenation. The mixtures of dianthracene isomers were analyzed
with respect to the anti/syn-ratio of the products by X-ray crystallography
and nuclear Overhauser effect spectroscopy. While for the 2,3-dimethylanthracene
the anti and syn isomers were formed in equal amounts, the anti dimers
are the major products in all other cases. A linear correlation (R2 = 0.98) between the steric size (Charton parameter)
and the isomeric ratio suggests that the selectivity is dominated
by classical repulsive steric effects. An exception is the iso-butyl
substituent that produces an increased amount of the syn isomer. It
is suggested that this is due to an exalted effect of London dispersion
interactions.

越来越多的证据表明，有机、有机金属及无机化学中庞大取代基的效应并非仅为排斥作用，由于伦敦色散相互作用（London dispersion interactions），其亦可产生吸引作用。本研究考察了蒽的2位与2,3位伯烷基取代基的尺寸，对[π4s + π4s]光诱导二聚反应的非对映选择性（anti与syn二聚体之比）的影响。蒽衍生物的合成可通过两条路径实现：一是2,3-二溴蒽与烷基硼酸的铃木-宫浦（Suzuki–Miyaura）偶联反应；二是先由2,3-二取代1,3-丁二烯与萘醌制备蒽醌类化合物，再经脱氢还原得到目标产物。通过X射线晶体学与核欧沃豪斯效应波谱（nuclear Overhauser effect spectroscopy），对二蒽异构体混合物的anti/syn产物比例进行了分析。对于2,3-二甲基蒽，anti与syn异构体的生成量相等；而在其余所有案例中，anti二聚体均为主要产物。取代基空间尺寸（查顿参数，Charton parameter）与异构体比例之间呈现线性相关（R² = 0.98），表明选择性主要由经典的排斥性空间位阻效应主导。唯有异丁基取代基例外，其可生成更多量的syn异构体，这一现象被认为源于伦敦色散相互作用的增强效应。